The present invention relates to cylinder liners for internal combustion engines containing a matrix based on aluminum alloys of high mechanical strength, obtained by powder metallurgy. It also concerns some of the methods of producing these cylinder liners.
Engine cylinder liners on aluminum are not new, but their use has always posed problems of compatibility between their working surface and the engine components, such as pistons, which are in contact with them. Attempts have been made to deal with the difficulties encountered in different ways, such as by providing a steel lining, or by coating the internal wall of the cylinder with harder metals such as iron or chromium, but it has not been possible to completely eliminate the difficulties.
Manufacturers then turned to alloys with better mechanical strength, such as hypereutectic aluminum-silicon alloys. However, the primary silicon crystals which appeared when the liner was molded were found to have an annoying tendency to score the surface of the pistons, due to their relatively large size and angular shape, and the surface of the pistons therefore had to be protected by a coating.
At that stage, since there was still a desire to enjoy some advantages provided by hypereutectic A-S alloys, attempts were made to change the structure of the alloys, particularly with respect of the grains of silicon, to try to give them the necessary compatibility without requiring subsequent surface treatments of the pistons. Among the attempts made the following should be mentioned:
Those methods consisting of changing the casting structure, such as those described in French Pat. No. 1 441 860, where an acid attack is made on the aluminummatrix so as to bring the silicon grains into relief. The grains are then polished. PA0 Those methods designed to give a new casting structure. This is the case in French Pat. No. 2 235 534, where the liner is molded under cooling conditions so that it does not have any primary silicon phase, but instead has fibrous or spheroidized particles with dimensions of less than 10 .mu.m. PA0 suitable hardness of the liner material to give it sufficient wear resistance while avoiding the appearance of scores; PA0 a relatively low coefficient of friction to facilitate movement of the components relative to one another; and PA0 a tendency of the materials not to stick together, in order to prevent any surface deterioration.
More recently, in their French Pat. No. 2 343 895, Applicants also turned to a new hypereutectic A-S structure but replaced the casting process with a process of extruding powders obtained by atomization. A method of this type in fact has the advantage of using powders formed at a very high rate of cooling, in which the primary silicon grains are relatively small and in any event, smaller than the size resulting from conventional casting. The size is not affected by the extrusion process and a new structure is thus obtained, with fine, well distributed silicon particles, and this considerably improves the compatibility of the liner with the piston.
However, in the course of tests carried out under particularly severe temperature conditions, the resultant liner is nevertheless found to deteriorate.
After carefully studying this phenomenon, Applicants discovered cause of the low resistance to be local adhesion between the liner and the piston.
They extended their research and reconsidered the criteria for good compatibility, namely:
On studying the various prior art solutions, Applicants concluded that, although the hardness function had been achieved fairly well with the primary silicon particles, and although the friction function had been achieved by adding certain lubricants such as graphite, a satisfactory solution to the non-sticking function had not yet been found among the means hitherto used.
For this reason Applicants paid particular attention to the problem of finding an appropriate non-sticking agent.